Social behaviors that impact human health, including risk-taking, aggression, and parental care, have a neuroendocrine basis that is both biologically based and well conserved across vertebrates. Animal models, particularly those in which definitive neuroendocrine and genetic links to social behavior have been described, are therefore valuable systems in which to study the hormones and genes that influence human social behavior. Ideally, such models would be easily studied in their natural habitats, reasonably similar to humans in their social behavior, genetically tractable, and inexpensive to work with. We have been studying the neuroendocrine and genetic bases of social behavior in an exceptionally promising model, the white-throated sparrow (Zonotrichia albicollis). This songbird, which is native to North America, has generated a great deal of interest among behavioral biologists because of a plumage polymorphism that predicts many aspects of an individual's social behavior. Dozens of behavioral studies conducted in the animals' natural habitat have established that individuals with a white-stripe (WS) on the crown are more competitive and aggressive, and birds with a tan stripe (TS) engage in more parental care. The color polymorphism is associated with a structural rearrangement on chromosome 2; all of the WS individuals have a copy of the rearranged chromosome (2m), whereas those of the TS morph are homozygous for the wild-type chromosome. We are currently mapping the 2m chromosome and have identified a set of promising neuroendocrine genes that are located within the rearrangement and may therefore contribute toward the behavioral phenotype and thus to risk-taking, aggression, and parenting. These genes, which include a gonadal steroid receptor, a steroidogenic enzyme, and a serotonin receptor subtype, are already suspected to play a role in social behavior in vertebrates, including humans. Here, we propose to evaluate these genes by identifying those (1) whose expression mirrors the behavioral polymorphism; (2) that may have organizational effects on the development of polymorphic behavior; and (3) that have been directly altered by the chromosomal arrangement in ways that may alter gene expression or function. Overall, the white-throated sparrow represents a unique and valuable opportunity for studies of the biological bases of social behavior because the behavioral differences between the morphs are already well-documented in free-living and laboratory populations, and the chromosomal inversion has been definitively linked to the behavioral polymorphism. Because the genes and pathways that regulate social behavior are conserved across species, the results of these studies will be applicable to understanding the mechanisms underlying aggressive and parental behavior in humans.